Using set_post_thumbnail inside save_post action gets overwrittenI was trying to set the Core post-thumbnail. I found my issue. I was running the function: delete_post_meta($post_id, '_thumbnail_id'), before I would set_post_thumbnail($post_id,$thumb_id); Apparently even though the delete was in the code before it still caused it to delete it from the DB. Very strange... I had it in there as I was worried I would be creating multiple _thumbnail_id meta entries for the same post.